The different molecular forms of Factor VIII antigen in platelets will be isolated using a combination of DEAE-agarose concentration, ultracentrifugation and preparative counter immunoelectrophoresis. The material so obtained will be analyzed on reduced SDS-polyacrylamide gels and the Factor VIII bands evaluated for relative amounts of PAS-positive material. The bands will be cut out and subjected to amino acid analysis and to quantitation of amino sugars. Antibody will be raised to the Factor VIII antigen forms isolated by these techniques. Platelet Factor VIII antigen will be compared to that in plasma by these criteria. The interaction of plasma Factor VIII with the platelet will be studied utilizing purified platelet Factor receptor (glycocalicin). BIBLIOGRAPHIC REFERENCES: Rothberger, H., T.S. Zimmerman, H.L. Spiegelberg and J.H. Vaughan. Leukocyte procoagulant activity. Enhancement of production in vitro by IgG and antigen-antibody complexes. J. Clin. Invest. (March 1977 issue), in press. Zimmerman, T.S., L. de la Pointe and T.S. Edgington. Interaction of Factor VIII antigen in hemophilic plasmas with human antibodies to Factor VIII. J. Clin. Invest. (April 1977 issue), in press.